1. Field of the Invention
This invention relates to a bottom feeding mechanism, in particular, to a mechanism for feeding media only one piece at a time to a functional unit, such as a printer.
2. Description of the Prior Art
An apparatus which peels off the bottom piece of a stack of media, for example, envelopes, and then feeds that piece to a functional unit to be processed is conventionally defined as a bottom feeder. The bottom feeder, in contrast with the top feeder which peels off the top of a stack of media, is capable of feeding media other than sheets of paper, say, envelopes. Commercially available bottom feeders have the following characteristics:
(1) The leading edge of the envelope stack rests on a rubber feeder roller. PA1 (2) A mechanical gate above the feed roller is adjustable by the operator to assure that the gap is wide enough to feed a single piece but not wide enough for more than one piece to go through the gate. PA1 (3) The roller is usually driven by a motor (typically, an A.C. motor) without proper feedback to control the velocity or position of the envelope. PA1 (1) The envelope itself is a paper product, it absorbs or releases moisture from and to the air. As a result two things change, its thickness and surface properties (i.e. frictional coefficient). PA1 (2) The envelope is also a pocket that traps air when stacked up collectively. The stack can be viewed as a group of compression springs connected in series. The bottom piece which is under the greatest load from above, is the thinnest and the top piece is the thickest. PA1 (1) Manual adjustment of the gate gap is not enough to assure uninterrupted operation. PA1 (2) Stack height is limited. PA1 (3) Taking three pieces per second and 200 pieces of stacked envelopes as an example, the time for depleting the stack is a little longer than a minute. Attended operation is almost mandatory and cost of operating the machine goes up.
The shortcomings of the above conventional bottom feeders can be understood by analyzing the physical properties of the envelope as follows:
Firstly, for a given envelope, thickness and friction coefficient between envelope and envelope/rubber feed roller are variable. A single gap adjustment is not sufficient to assure trouble free operation. In practice, the stack is limited in height, which may be two to three inches or approximately one to two hundred envelopes in business size. The tuning of the gate gap is time consuming and usually done on a trial error basis.
The second shortcoming is the prime mover itself. For an A.C. or a D.C. motor without proper feedback, the rotor speed depends on its payload. If the stack is high, the rotor is subjected to a greater retarding torque, hence its speed is less and gradually increases as the stack gets lower. A variable speed output from a high speed feeder could be potentially trouble-inviting. Paper jam is a common unpleasant experience for everyone in this field. To avoid unacceptable speed variation and to accomplish a very high burst speed (10,000 pieces up per hour), a very powerful motor is usually used. The potential trouble is reduced but not eliminated, the motor cannot be stopped quickly enough to avoid massive jams.
In summary, the weaknesses of the conventional bottom feeders are listed as follows: